In U.S. Pat. No. 3,794,468, issued Feb. 26, 1974, in the name of Ronald J. Leonard for "Mass Transfer Device Having a Wound Tubular Diffusion Membrane" and assigned to the assignee of the present invention, a mass transfer device is disclosed. The mass transfer device disclosed in that patent is made by winding a length of a hollow tubular conduit of semipermeable membrane about a core to form a wound bundle similar to packages of kite string, in which individual adjacent windings in the same layer are generally parallel to each other, but individual adjacent windings of the conduit in adjacent layers define an angle to each other. The device of that patent is assembled by winding a length of the hollow tubular conduit of semipermeable membrane about a core in a plane which defines an acute angle to the longitudinal axis and intersects both ends of the core. Simultaneously, the core is rotated about its longitudinal axis, or the plane of winding is correspondingly rotated to the same effect, to laterally displace on the core each winding of conduit from its immediately preceding winding. The resulting wound structure allows a low cost construction of a high performance mass transfer device. The flow pattern is around the circumference of the cylindrical element formed by the winding process. However, as the flow gas around the circumference, it will encounter flow paths of different length. Additionally, since the core is rotated at a constant rotation velocity and the angular velocity of the wind is constant, the void fraction of the resulting structure increases radially outwardly of the bundle. In other words, the void fraction near the outside of the bundle will be greater than the void fraction on the inside of the bundle. As used herein, the term "void fraction" connotes the ratio of space to space and fiber, whereby a greater void fraction means more space.
We have discovered that performance of the mass transfer device is decreased if the void fraction near the outside of the bundle is substantially greater than the void fraction near the inside of the bundle. If the mass transfer device is an oxygenator, for example, having a void fraction on the outside that is substantially larger than the void fraction on the inside results in a much larger blood flow rate on the outside because there is more resistance on the inside. As the void fraction on the outside becomes smaller, there is less flow rate on the outside. However, since the flow path on the outside is inherently longer than the flow path on the inside, if the void fraction were equal throughout the bundle, the flow rate on the outside would not be equal to the flow rate on the inside but the blood flow would be unequal because of the longer flow path on the outside.
We have discovered than an optimum hollow fiber oxygenator comprises a bundle in which the flow rate is substantially constant throughout the bundle, the blood outlet saturation is substantially constant throughout the bundle, and the void fraction increases slightly in the radial outward direction of the bundle. The present invention provides a novel process for obtaining this optimum device and also concerns a novel device having these optimum properties.
Although the illustrative embodiment of the invention relates to a hollow fiber mass transfer device such as an oxygenator, it is to be understood that the present invention may be applicable to other devices using other fibers. For example, the present invention could be used with non-permeable tubing such as employed in a heat exchanger. Further, the present invention could be used with an absorption filter, using an absorbant filter material such as polyesters or natural fibers. Other impregnated fibers could be employed in connection with the present invention.